1. Field of the Invention:
This invention relates to a recovery circuit for recovery a carrier from a phase modulated signal, and more particularly to a carrier recovering circuit which employs an adaptive line enhancer.
2. Description of the Related Art:
Conventionally, as a technique for recovering a carrier from a phase modulated wave using an adaptive line enhancer, a carrier regeneration circuit is disclosed in Japanese Patent Laid-Open Application No. 198205/1990 or No. 61534/1992. In the carrier recovery circuit which employs an adaptive line enhancer, a signal obtained by removing modulation components from a phase modulated signal is inputted to the adaptive line enhancer. In the adaptive emission line emphasizer, the signal successively passes through a correlation separator and an adaptive filter, whose coefficients are controlled by an adaptive algorithm circuit, to be outputted as an extracted carrier. Then, the extracted carrier is synchronized to obtain a recovery carrier. The carrier recovery circuit which employs such an adaptive line enhancer as described above can provide with superior characteristics in terms of the synchronization characteristic at a lower signal to noise ratio, improve the pull-in characteristic over a wide range, reduce the time required for pull-in and so forth compared with a former technique which employs a phase locked loop.
The carrier recovery circuit employing an adaptive line enhancer, however, has a problem in that it involves a great amount of renewal calculation processing of tap coefficients, which are coefficients for individual taps required for the coefficient control of the adaptive filter, and this makes high speed pull-in at a high transmission rate difficult. Further, where the carrier recovery circuit is implemented based on real time processing, a limitation in processing capacity of a processor such as a digital signal processor limits the number of taps of the adaptive filter. Particularly, where a signal of a high transmission rate is handled, the carrier regeneration circuit suffers from the problem that the band of the adaptive filter cannot be made sufficiently narrow because of the restriction on the number of taps and thus high synchronization stability cannot be assured at a low signal to noise ratio.